The present invention relates to a hydraulic control system for an automatic transmission.
U.S. Pat. No. 4,271,939 discloses a hydraulic control system for an automatic transmission provided with a lock-up clutch. The lock-up clutch is disposed within a torque converter and functions to mechanically connect in impeller with a turbine. In detail, there is arranged within the torque converter a lock-up clutch piston which defines a lock-up release chamber. When the lock-up clutch piston assumes a clutch engaged position, the impeller is mechanically connected with the turbine, whereas when it assumes a clutch released position, the impeller is mechanically disconnected from the turbine, thus allowing the torque converter to assume a torque converter state. The lock-up clutch piston is biased to the clutch engaged position by actuating pressure within a converter chamber of the torque converter (i.e., the actuating pressure being created within the working chamber owing to hydraulic power transmission by the impeller, turbine and stator) when oil is discharged from the lock-up release chamber. When the oil pressure is applied to the lock-up release chamber, the lock-up clutch piston is moved against the actuating pressure in the working chamber of the torque converter until the clutch released position. The lock-up chamber and the working chamber of the torque converter are supplied with oil from the same oil conduit. But, since oil is supplied via an orifice to the working chamber of the torque converter and oil is discharged therefrom at a constant rate, the oil pressure within the lock-up release chamber is kept slightly higher than that in the working chamber of the torque converter, thus ensuring release of the lock-up clutch. The oil pressure within the working chamber of the torque converter is supplied from a line pressure regulator valve. Viz., when the line pressure regulator valve has reached an equilibrium state thereof and the line pressure has increased beyond a predetermined value, a port connected with an oil conduit communicating with the working chamber of the torque converter is opened, admitting discharged oil to the working chamber of the torque converter. When the amount of discharged oil via this port is not enough to regulate the line pressure, the oil is discharged also through a drain port of the line pressure regulator valve. A hydraulic control system for an automatic transmission including the above mentioned line pressure regulator valve presents the following problem. Since the line pressure regulator valve admits a superabundant amount of oil to the torque converter only when the line pressure has reached the predetermined value as described above, when the line pressure is lower than the predetermined value and thus the amount of discharged oil is not sufficient, the oil hardly flows into the working chamber of the torque converter, thus creating no difference in pressure between oil pressure in the working chamber and that in the lock-up chamber. This causes the lock-up clutch to remain in a slightly engaged state against a demand that the lock-up clutch be released. When the vehicle moves off from a standstill, the line pressure regulator valve cannot discharge enough flow of oil toward the torque converter because an oil pump driven by an engine rotates at low speeds. If the lock-up clutch is slightly engaged when the vehicle is to move off from a standstill, the vehicle cannot start smoothly and with sufficient acceleration. One measure to solve this problem is to use an oil pump having a large capacity. But this presents a problem of how to find an installation space because of its large size. Another problem is that a loss in torque loss for driving the oil pump is large.